Capillary instability of a two-layer annular film: an airway closure model

Capillary instability of a two-layer liquid film lining a rigid tube is studied computationally as a model for liquid plug formation and closure of human airways. The two-layer liquid consists of a serous layer, also called the periciliary liquid layer, at the inner side and a mucus layer at the outer side. Together, they form the airway surface liquid lining the airway wall and surrounding an air core. Liquid plug formation occurs due to Plateau–Rayleigh instability when the liquid film thickness exceeds a critical value. Numerical simulations are performed for the entire closure process, including the pre- and post-coalescence phases. The mechanical stresses and their gradients on the airway wall are investigated for physiologically relevant ranges of the mucus-to-serous thickness ratio, the viscosity ratio, and the air–mucus and serous–mucus surface tensions encompassing healthy and pathological conditions of a typical adult human lung. The growth rate of the two-layer model is found to be higher in comparison with a one-layer equivalent configuration. This leads to a much sooner closure in the two-layer model than that in the corresponding one-layer model. Moreover, it is found that the serous layer generally provides an effective protection to the pulmonary epithelium against high shear stress excursions and their gradients. A linear stability analysis is also performed, and the results are found to be in good qualitative agreement with the simulations. Finally, a secondary coalescence that may occur during the post-closure phase is investigated.

Experimental characterization of mixing and flow field in the liquid plugs of gas–liquid flow in a helically coiled reactor

Flow mixing of two miscible liquids with the addition of gas bubbles is a process often found in industrial chemical apparatus for the production of primary matter. The ongoing optimization of such processes also involves the transformation of batch to continuous mode operation. In that case, the use of helically coiled tubes is an interesting alternative, since those reactors have narrow residence time distributions, very good radial mixing properties and excellent mass transfer can be realized between gases and liquids. For these reasons, in this study the mixing of two miscible liquids with addition of air bubbles in gas–liquid flows has been characterized in a horizontal helically coiled reactor in the laminar flow regime at $${\text{Re}}_{{{\text{total}}}} = 300 \ldots 1088$$ Re total = 300 … 1088 . Eight different superficial liquid velocities and five superficial gas velocities were investigated. In order to characterize mixing in the liquid plugs between two bubbles, laser-induced fluorescence of resorufin was used and particle image velocimetry has been employed to characterize the flow field. Pseudo-3D-visualizations of the resorufin concentration and the Q-criterion, representing the mixing efficiency and vorticity, respectively, were established for individual liquid plugs from the time-resolved measurement results. A time-resolved mixing coefficient, as well as a mean mixing coefficient obtained from multiple liquid plugs, is calculated from the fluorescence images for all examined flow conditions. The experimental results clearly show an increase in the mixing coefficient compared to single-phase conditions, caused by the bubbles. However, distinct mixing pattern, depending on the flow structure, can be recognized on different locations inside the liquid plug. Compared to a stationary case without air bubbles, mixing is worse behind the bubbles and increases inside the plug, reaching a maximum mixing coefficient in front of the next bubble. Overall the mixing coefficient is always increased by the presence of the bubbles. Pseudo-3D-visualizations of the Q-criterion and the vorticity show the presence of secondary vortices right in front of the bubbles, shifted to the outer tube walls, and in addition to the steady Dean vortices. In small plugs, these secondary vortices appear in the whole plug and increase the mixing coefficient drastically. Graphical Abstract

Research of the fluid flow regime during the milk pipeline washing

The aim of the research was to determine the change in the volume of fluid slug moving in the milk pipeline for calculating the initial length of the fluid slug and thus the air slug length. This allows to control the fluid and air inlet valve during circulatory washing of milking equipment with milk line and provides a stable plug flow during washing. The article deals with the problems of formation of a wave flow and plug flow regime in a milk line in dependance to the degree of filling and shear stress on the surface of the phases. As the result, the dependence curves have been obtained. They show that when the degree of filling the milk line with liquid is from 0.4d to 0.6d, the probability of slug formation decreases. To determine the loss of a fluid slug volume moving in a milk line, mathematical calculations with the use of the boundary layer theory have been carried out. As the result, the curves of the time of fluid and air intake into the system depending on the length and diameter of the milk pipeline have been obtained. These engineering calculation methods allow to set the operating parameters of the valves for fluid and air inlet into the system during washing. To calculate the parameters of pipelines with an internal diameter of 48, 63, 70 and 98 mm often used in milking equipment, a Delphi-based program was written. The article provides an example of calculating the time of opening and closing the valve for the inlet of fluid and air during washing for a milk line with 48 mm pipe diameter, 120 m length and pressure in the system of 48 kPa. Experimental studies have confirmed the reliability of the calculations, the loss of the liquid plug length for the milk pipeline Ø50. 8 mm is on average 5 cm per 1 meter of the fluid path.

Numerical study on gas–liquid two-phase flow and mass transfer in a microchannel

A numerical study of the gas–liquid two-phase flow and mass transfer in a square microchannel with a T-junction is carried out in this work. Through numerical simulation methods, the flow patterns of bubble flow, slug flow and annular flow are determined. By proposing a new flow pattern conversion relationship with different media and different speeds, 100 sets of CO2-water flow patterns and 100 sets of CO2-ethanol flow patterns are obtained. The effects of surface tension on flow pattern, bubble length and liquid plug length are studied. The pressure distribution and pressure drop are analyzed, and mass transfer is obtained through slug flow simulation, and the influencing factors of gas–liquid mass transfer are studied. The results show that the effect of surface tension on the length of the bubble and the length of the liquid plug is completely opposite, the pressure distribution is stepped, and the pressure drop increases with the increase of the gas–liquid velocity. In addition, it was found that the volumetric mass transfer coefficients of the bubble cap and the liquid film gradually decreased with time, and eventually stabilized. The increase in bubble velocity accelerates the mass transfer rate, while the increase in unit cell length slows the mass transfer rate. However, the influence of film thickness and liquid film length on mass transfer varies with time.

Modeling for Fluid Flow and Heat Transfer in Closed Loop Pulsating Heat Pipe

Numerical modelling of multi turn Closed Loop Pulsating Heat Pipe (CLPHP) is presented in this paper for ethanol as working fluid. Modelling is carried out for 1mm and 2mm ID PHP for different number of turns, different orientations and at constant wall temperature boundary conditions. Momentum and heat transfer variations with time are investigated numerically solving the one dimensional governing equations for vapor bubble and liquid plugs. Evaporation and condensation takes place by heat transfer through liquid film present around the vapour bubble. The code takes into account the realistic phenomena such as vapour bubble generation, liquid plug merging and super heating of vapor bubbles above its saturation temperature. During merging of liquid plugs, a time step adaptive scheme is implemented and this minimum time step was found to be 10−7 s. Nature of flow is investigated by momentum variation plot. Model results are compared with the experimental results from literature for nine different cases. Maximum variation in heat transfer for all these cases is found to be below ±34%. Keywords: Closed Loop Pulsating Heat Pipe, Liquid Plug, Plug momentum, Vapor Bubble, Heat Transfer, Thin Film Evaporation and Condensation

Magnetohydrodynamic Moving Liquid Plug Within a Microchannel: Analytical Solutions

The wide applications of plug flows in microscale in science and engineering help them attract a great deal recent interest. An analytical study is undertaken here to study the impacts of a transversely applied external uniform magnetic field affecting the motion of liquid in the plug in terms of hydrodynamic mixing properties. The well-known symmetric vortex structure occurring in a long plug with moderate aspect ratio is observed to be preserved, while the recirculation phenomenon is highly affected by the action of the magnetic field. The decelerating feature of Lorentz force on the liquid motion is illuminated by reducing the strength of the recirculating vortex moving towards the upper and lower walls. The effects of magnetic field on the flow resistance of the liquid plug as well as on the plug circulation rate and on the axial flux are also clarified. The liquid plug considered here is shown to be fully consistent with the continuous liquid flow in a channel whose exact solution is further extracted.